Project Summary: Type 1 diabetes mellitus (T1D) results from reduced total number or diminished function of insulin-producing beta-cells within the pancreatic islets of Langerhans. How lipid metabolism influences the ability of the islet beta-cells to secrete insulin, regulate glucose metabolism, and proliferate has been largely studied almost exclusively in isolated islets using acute in vitro approaches. However, studies conducted in vitro have led to differential and often conflicting results. Herein, we propose two novel mouse models that allow us to address the role of lipid metabolism from two critical organelles to investigate targeted and specific outcomes in vivo. This approach will allow us to answer the fundamental question: does inhibition of fatty acid oxidation in pancreatic tissue, including islet beta-cells, contribute to alterations in beta-cell function observed during aging, insulin resistance, and diabetes? To address this question, we will use mice lacking either carnitine palmitoyltransferase-1a (CPT-1a; Aim 1), a mitochondrial gene, or Pex5 (peroxisomal gene; Aim 2) in pancreatic and islet tissue. In Specific Aim 1, we will use the pancreas-specific deletion of the Cpt1a gene in combination with low-fat and high-fat diets to determine whether limiting fatty acid oxidation enhances glucose metabolism to support insulin secretion. In this experiment, we will also assess whether fatty acid oxidation is critical for the expansion of beta-cells observed during states of insulin resistance in vivo. In Specific Aim 2, we will limit fatty acid oxidation in the peroxisomal compartment, which regulates the metabolism of very long chain fatty acids, to determine whether this enhances beta-cell function or promotes lipid storage to generate metabolic stress and key features of lipotoxicity. These novel genetic approaches will provide critical insights into the role of lipid oxidation during aging and obesity. Understanding the principles and mechanisms underlying fatty acid oxidation on islet biology in vivo have important implications for suppressing, treating, and curing obesity- and aging-associated metabolic diseases.